Manufacture of hollow clayware articles

ABSTRACT

A method of sizing a clayware article comprising internally fitting the article before firing with a mandrel having a coefficient of expansion greater than the coefficient of expansion of the clay when fired, firing the article to a temperature at which the clay becomes plastic and conforms to the mandrel, allowing the fired article to cool whereby the mandrel becomes separated from the article by virtue of the differential shrinkage of the fired clay and the material of the mandrel, and removing the mandrel from the article.

United States Patent 1 Booth July 9,1974

[ MANUFACTURE OF HOLLOW CLAYWARE ARTICLES [75] Inventor: John Frederick Booth, Sheffield,

England [73] Assignee: The l-lepworth Iron Company Limited, Stocksb'ridge, England 221 Filed: June 14, 1973 I [2]] App]. No.: 369,881

[30] Foreign Application Priority Data FOREIGN PATENTS OR APPLICATIONS 1,086,166 7/1960 Germany 432/358 Primary Examiner-John .l. Camby Attorney, Agent, or FirmCushman, Darby &

Cushman [5 7] ABSTRACT 10 Claims, 2 Drawing Figures June I5, 1972 Great Britain 28091/72 52 us. or. 432/5, 432/258 [51 l Int. Cl. F27b 19/04 [58] Field of Search 432/2, 4, 5,258

[56] References Cited UNITED STATES PATENTS l,920,589 8/1933 Payne 432/258 SIZE PATENTEDJUL 9814 TEMPERATURE UP M, TEMPERATURE DUWN.

MANUFACTURE OF HOLLOW CLAYWARE ARTICLES ofexpansion greater than the coefficient of expansion of the claywhen fired, firing the article to a temperature at which the clay becomes plastic and conforms to the mandrel, allowing the fired article to cool whereby the mandrel becomes separated from the article by virtue of the differential shrinkage of the fired clay and the material of the mandrel, and removing the mandrel mm the article.

The method may include the step of drying the article to a predetermined hardness before firing.

The mandrel may be fitted in the article at any stage before the clay becomes plastic, but is preferably fitted after drying the moulded article. The insertion and removal of the mandrel, referred to as mandrel cycling, may take place entirely within the firing kiln.

The relative sizes of the mandrel and the article should be such that when the clay becomes plastic during the firing step, it shrinks onto the mandrel, thereby closely conforming to the mandrel. Contrary to expectations, the clay is not prone to cracking in spite of conforming closely to the mandrel whilst it is in the plastic condition. Immediately, when there is a drop in temperature, the differential shrinkage causes the mandrel to separate from the article.

The method may further include the step of freezecooling the article, after it has been fired, by blowing air into the firing kiln.

The accuracy of sizing is dependent, naturally, on the accuracy of the mandrels. These may be formed of high heat resistant material, e.g., refractory metals, or heat resistant alloys such as those in the Nimonic (Registered Trade Mark) range, which, even after considerable repeated use, accurately maintain their dimensions and shape.

Mandrel materials which can be used to advantage are heat-resisting steels characterised by highresistance to oxidation or scaling and the maintenance of dimensions under stress at elevated temperatures and through thermal cycling.

The scaling, or oxidation-resistance, of heat-resisting steels is due primarily to the element chromium and, to a lesser extent, to silicon. The chromium in this class of steels ranges from approximately 8% to whilst silicon may be present in additions up to 4%. The advantage of nickel, from the point of view of oxidation resistance, is only apparent when the nickel content is high, i.e. when the iron is largely replaced by the nickel.

Another property required of heat-resisting steels is the maintenance of dimensions under stress at elevated temperatures, in other words creep resistance. Chromium confers good scaling resistance but does not contribute to creep resistance. The most useful element is the. nickel which, when added to a chromium steel in sufficient quantities, makes the steel austenitic in bait and tungsten.

A particularly successful heat-resisting steel has been found to be an iron/chromium ferritic alloy containing about 28% chromium. This particular type of alloy was selected as being especially suitable for resisting kiln conditions of temperatures up to l,l00 C in an atmosphere where clayware items are being fired, where high sulphur and oxidation is prevalent and where strength, although not of primary importance, must be given consideration. Another advantage with this material is that it is magnetic and this property can be utilised in the insertion of blanks into the clay. The material also has cost advantages, being cheaper than the high nickel content steels.

The method is particularly suited for the sizing of clayware pipes. Thus the method may include the step of inserting a mandrel internally of each end of the pipe for accurately detemiining the dimensions of the ends .of the pipe when fired, the process also having the effect of indirectly determining the external dimensions of the pipe ends. This permits the manufacture of pipes to close tolerances, which is desirable particularly in the cases of pipes of the spigot and socket coupling type where close fitting couplings are advantageous.

Conveniently, a spigoted and socketed pipe moulded from clay is stood on its spigoted end over a mandrel supported by a setting ring having a larger outer diameterso as to support the pipe also, and a mandrel is placed in the socketed end. A plain-end (i.e., nonsocketed) pipe may be stood on either end over a mandrel supported by a setting ring having a larger outer diameter so as to support the pipe also, and a mandrel inserted in the upper end with a flange'to rest on the upper end face so as to support that mandrel. Conveniently both mandrels may be provided with flanges so that either can be used at either end, and both mandrels (and also a mandrel for the spigoted end of a spigoted and socketed pipe) may be provided with a slight taper the larger end of which is adjacent the flange, to facilitate insertion in pipe ends or setting of pipe ends over the mandrels.

The invention will now be described by way of example, with reference to the accompanying drawings in which:

,FIG. 1 shows a sectional side view of a clay pipe supported on a kiln car before firing and with a mandrel fitted in each of its ends; and

FIG. 2 is a schematic representation of the expansion and contraction curves of the pipe and the mandrels through the firing and cooling stages.

Referring now to FIG. 1, there is shown a clayware pipe 10 having a spigot end 12 and a socket end 14. The pipe 10 is supported upright with the spigot end 12 downwards on a kiln car 16 (only partially shown) by means of a setting ring 18. A spigot mandrel 20 is fitted with clearance in the spigot end 12 of the pipe 10 by supporting it on a second setting ring 21 which is also supported on the kiln car 16. A socket mandrel 22 is fitted with clearance in the socket end 14, the socket end 14 being slightly wider than the body portion of the pipe, thereby forming an internal shoulder 24 which supports the socket mandrel 22.

After moulding or extrusion of the pipe and either before or after mandrel insertion the pipe is dried at a temperature of up to 200 C until the moisture content has been reduced to approximately 1%. The drying process may be carried out in separate drying chambers or on a roller drier conveyor. Alternatively the pipes may be set directly on the kiln car after extrusion and the car then taken through a drying kiln. The latter method reduces the number of handling operations. Another variation would be first to dry the pipes to an intermediate degree of dryness and then to place them on the kiln car for further drying.

The pipe is then placed on the kiln car 16 with the mandrels 20 and 22 in position as shown in FIG. 1. The inside diameters of the pipe ends 12 and 14 are slightly larger than the outside diameters of the mandrels.

The mandrels 20 and 22 are made from steel having a high resistance to oxidation or scaling, and which is able to maintain its shape under stress at elevated temperatures and through thermal cycling. A steel found to be particularly suitable was an iron/chromium ferritic alloy having the following composition:

Fe -s'i' S r cr rri' 69.12 0.87 0.86 0.34 .015 .028 28.11 0.66

This alloy has the additional advantage that it is magnetic, which property may be advantageously utilized in inserting the mandrels into or withdrawing the mandrels from the pipe.

After the pipe has been placed on the kiln car 16, the car is moved into a firing kiln and the temperature raised to approximately l,040 C to fire the clay.

Thereafter the ppes are freezecooled by blowing air into the kiln (in known manner) whereby their temperature is reduced to approximately 700 C in a period of 4 hours. As a result of the greater coefficient of expansion of the mandrels with respect to that of the fired clay, the mandrels will have contracted more than the pipe and will, at this stage, have separated from the pipe. The mandrels are then removed from the pipe.

Referring now to FIG. 2, the chain dotted line 30.1 indicates the expansion curve of one of the mandrels as the firing process proceeds from left to right in the graph. The expansion continues until the maximum temperature 32 is reached and thereafter contracts along chain dotted line 30.2 as the pipe is freezccooled.

The solid line 34.1 indicated the expansion of the pipe as it is heated during the first part of the firing process. The initial clearance between the pipe and the mandrel is sufficient to prevent the mandrel from expanding to a larger diameter than that of the pipe during this first part. At a temperature just lower than the maximum the clay starts to fuse, causing the pipe to contract along the solid line 34.2. Contraction of the clay begins with the breakdown of the micaceous clay mineral and the liberation of glass forming and fluxing oxides. The clay continues to contract until it has shrunk into contact with the mandrel. When fusing commences, the clay becomes plastic and is therefore plastically deformed to a shape corresponding closely with that of the mandrel. Thereafter the clay is forced to expand slightly together with the mandrel along the solid line 34.3 until the maximum temperature 32 is reached.

While the pipe is being freeze-cooled the pipe contracts along solid line 34.4. As is shown in the graph the mandrel contracts more than the tired pipe causing the mandrel to separate from the pipe so that it may easily be removed.

The dotted lines 34.5 and 34.6 indicate the contraction of the pipe which would occur if the mandrel were not present.

As an example a number of clayware pipes extruded from a clay mixture consisting of a blend of equal parts of shales and mudstones were treated by the method described above. The sizes of pipes and of the mandrels used are shown in the following table:

Socket size Spigot size (ins.) (ins) Design size 10.926 (l.D.) 10.460 (O.D.) Mandrel size,

at 20 C 10.860 (O.D.) 9.000 (l.D.) at 1050C 11.010 (O.D.) 9.120 (l.D.) Clay sizes,

green 11.450 (l.D.) 11.160 (O.D.) fired 10.930 (l.D.) 10.450 (O.D.)

The pipes were fired at a maximum temperature of 1,040 C 1 10 C and a circularity with a tolerance of 0.030 inch in the final product was obtained.

Although only the inner diameters of the pipes are directly determined by the method described above, the outside diameters, in particular of the spigot ends, are also accurately determined.

It is important that contact between the clay and mandrel should be established whilst the clay is in its plastic state by shrinkage of the clay onto the mandrel. To achieve this result, the initial dimensions of the clayware article and mandrel and the intial clearances therebetween are important and will depend on the characteristics of the particular clay and mandrel materials. These clearances and dimensions should in each case be determined from expansion curves of the type shown in FIG. 2 for the particular materials being used.

1 claim:

1. A method of sizing a clayware article comprising internally fitting the article before firing with amandrel having a clearance therefrom sufficientto prevent the mandrel from expanding to a larger outer diameter than the internal diameter of the article where the mandrel is fitted until the article becomes plastic during firing, the mandrel having a coefficient of expansion greater than the coefficient of expansion of the clay when fired, firing the article to a temperature at which the clay becomes plastic and conforms to the mandrel, allowing the fired article to cool whereby the mandrel becomes separated from the article by virtue of the differential shrinkage of the fired clay and the material of the mandrel, and removing the mandrel from the article.

2. A method as claimed in claim 1, which includes the step of drying the moulded article to a predetermined hardness before firing.

3. A method as claimed in claim 2 wherein the mandrel is fitted after drying the moulded article to a predetermined hardness.

4. A method as claimed in claim 2 wherein the insertion and withdrawal of the mandrel takes place during the firing step.

5. A method as claimed in claim 1 wherein the mandrel is formed from a steel including between 8 to 30% chromium.

6. A method as claimed in claim 5 wherein the mandrel is made from an iron/chromium ferritic alloy containing about 28% chromium.

7. A method as claimed in claim 5 wherein the man- 9. A method as claimed in claim 1 in which the article drel material contains up to 4% silicon. after firing is cooled, by forced air cooling.

8. A m h as claimed n laim 5 h r n h 10. A method as claimed in claim 1, wherein the artidrel is made of a steel having the following percentage l i a i a d wherein a mandrel is inserted intercomposition Fe 69.12, C 0.87, Si 0.86, Mn 0.34, S 5 ll f a h nd of the pipe, 0.015, P 0.028, Cr 28.11, Ni 0.66. 

1. A method of sizing a clayware article comprising internally fitting the article before firing with a mandrel having a clearance therefrom sufficient to prevent the mandrel from expanding to a larger outer diameter than the internal diameter of the article where the mandrel is fitted until the article becomes plastic during firing, the mandrel having a coefficient of expansion greater than the coefficient of expansion of the clay when fired, firing the article to a temperature at which the clay becomes plastic and conforms to the mandrel, allowing the fired article to cool whereby the mandrel becomes separated from the article by virtue of the differential shrinkage of the fired clay and the material of the mandrel, and removing the mandrel from the article.
 2. A method as claimed in claim 1, which includes the step of drying the moulded article to a predetermined hardness before firing.
 3. A method as claimed in claim 2 wherein the mandrel is fitted after drying the moulded article to a predetermined hardness.
 4. A method as claimed in claim 2 wherein the insertion and withdrawal of the mandrel takes place during the firing step.
 5. A method as claimed in claim 1 wherein the mandrel is formed from a steel including between 8 to 30% chromium.
 6. A method as claimed in claim 5 wherein the mandrel is made from an iron/chromium ferritic alloy containing about 28% chromium.
 7. A method as claimed in claim 5 wherein the mandrel material contains up to 4% silicon.
 8. A method as claimed in claim 5 wherein the mandrel is made of a steel having the following percentage composition Fe 69.12, C 0.87, Si 0.86, Mn 0.34, S 0.015, P 0.028, Cr 28.11, Ni 0.66.
 9. A method as claimed in claim 1 in which the article after firing is cooled, by forced air cooling.
 10. A method as claimed in claim 1, wherein the article is a pipe and wherein a mandrel is inserted internally of each end of the pipe. 